A gas measuring apparatus off this kind for making a paramagnetic determination of oxygen is disclosed in U.S. Pat. No. 2,689,332. This apparatus includes a plurality of chambers which are each filled with the gas to be investigated having paramagnetic characteristics. A specific number of measuring chambers with a like number of reference chambers is guided along in pairs between the pole shoes of an even number of permanent magnets. Changes in the permeability of the cuvette material during rotation are compensated because of the series connection of the measuring field coils about the permanent magnets. This is so because the generated flux changes are opposite in phase. Possible changes of the permeability, which are still not compensated, can be compensated by applying small quantities of paramagnetic material to the cuvette disc. Suitable material for this purpose is red iron oxide.
A disadvantage in this known apparatus is the complex determination of suitable locations for the application of the additional paramagnetic material and the control as to whether the quantity of materials is adequate. Inconsistencies which remain must be compensated by downstream signal processing circuits.
A further apparatus of the type described above is disclosed in U.S. Pat. No. 4,950,984. The apparatus can be utilized for gas analysis as well as for monitoring waste water.
In the known apparatus, a disc-shaped cuvette rotates in a magnetic field which is generated by permanent magnets and which penetrates the cuvette. The measuring chambers of the cuvette comprise simple breakthroughs in the disc. The hollow spaces defined in this manner are flushed by the measuring gas to be investigated. The measuring chambers are separated from each other by a strip made of the material of the cuvette disc. This strip has a width which is more or less wide. This strip of material serves as a reference chamber filled with the material of the cuvette disc.
During the rotation of the cuvette, the measuring chambers for the gas to be measured or the cuvette disc material is passed by the magnetic field sources in dependence upon the number of measuring chambers. If a measuring gas such as oxygen is in the measuring chamber, then the paramagnetic characteristics of the measuring gas change the magnetic flux through the measuring chamber whereby an electric induction field is generated in a measuring field coil surrounding the magnetic field source.
It is desirable that disturbance signals are eliminated to the extent that even the smallest quantity of measuring gas can be detected without difficulty in order to obtain a measuring apparatus having the highest possible sensitivity. In addition to the suppression of disturbance signals, it is also necessary to have a stable zero value calibration at which the so-called zero signal is determined. This can be achieved in a simple manner in that, for example, nitrogen is used as a calibrating gas so that a paramagnetically generated induction signal is precluded. The calibration signal obtained in this manner however still contains a disturbing base signal. The cause of this is that the cuvette disc has different material thicknesses; that is, in the region of the measuring chamber, the measuring gas charge and possible thin chamber walls are to be considered and, in the area of the reference chamber region, the full material thickness of the cuvette disc or a reference chamber having a different gas content as reference gas is to be considered. In all cases, the thickness difference between the material of the reference chamber and of the measuring chamber is considerable so that the base signal is larger by a multiple than a minimum useful signal for the oxygen portion. The minimum useful signal is detectable because of the configuration of the apparatus.
As a rule, the cuvette disc is made of a diamagnetic material which can be magnetically neutralized by admixing paramagnetic or ferrimagnetic material. Even with this configuration, this method is complex and a cuvette disc produced in this manner cannot be changed subsequently in its magnetic characteristics with respect to the suppression of the base signal. Even if the neutralized diamagnetic disc itself would supply only a slight amount to the base signal, the fact would nonetheless remain that a disturbing base signal is generated because of the considerable thickness differences between the measuring gas and the reference material.